1. Field of the Invention
The present invention relates generally to video-recorders/reproducers and more particularly to an improvement therein permitting the slow-motion reproduction of video tapes recorded in a segmented helical-scan format.
2. Description of the Prior Art
Although of broader application, the preferred embodiment of the present invention employs a prior art, EIAJ-2, U-Matic Video-Tape Recorder/Reproducer of the type which is designated 2860 by the Sony Corporation and which is modified according to the teachings of my now U.S. Pat. No. 4,214,259.
Unmodified, the above-mentioned prior-art recorder/reproducer has modulation circuitry of the type which is known as color-under for encoding for recording a video signal. This modulation circuitry develops a pair of signals by separating the luminance (black and white) and chrominance (color) portions of the video signal. The amplitude of the luminance signal is used to modulate the frequency of an oscillator to develop a frequency modulated signal which deviates from approximately 3.5 to 5.5 megahertz. The chrominance signal, which is in the form of double-sideband suppressed-carrier modulation of a 3.58 megahertz carrier, is mixed with a suitable frequency local oscillator signal to translate the frequency of the chrominance signal to a carrier frequency of 688 kilohertz. Finally, the translated signal is combined with the frequency modulated signal to develop a color-under signal suitably encoded for recordation by a scanner upon a length of three quarter inch wide magnetic recording tape.
The scanner is comprised of a pair of coaxially-disposed cylindrically-shaped drums including a lower, rigidly mounted drum for guiding the tape and a rotatable upper drum for carrying a pair of diametrically-opposed peripherally-mounted magnetic recording/reproducing heads driven by the color-under signal. Rotational drive for the upper drum is obtained from a motor which is so coupled to the upper drum by a pair of pulleys and a belt as to drive the drum at a rotational rate in excess of 1800 r.p.m.
The rotational rate of the upper drum is controlled by a servo loop that includes a drum-speed-sensing tachometer for developing a signal proportional to the rotational rate of the drum, a brake for slowing the drum and a drum-servo block. The drum-servo block responds to the frequency of the tachometer signal and to the frequency of the horizontal-synchronization portion of the video signal and develops a brake-driving signal for synchronizing the speed of the drum with the horizontal rate.
From a video cassette in which it is stored, a portion of the magnetic recording tape is drawn partially around the scanner by the combination of a capstan and pinch roller before being returned to the cassette. The tape wraps just over 180 degrees of the scanner in helical fashion such that as the scanner rotates, the drum carried heads define video paths, or tracks, which extend partially across a central portion of the tape at an acute angle with respect to the length thereof. With each revolution of the upper drum, synchronized by the drum servo loop, each of these heads records one field of the color under signal on a respective video track of the tape.
The capstan is driven by a capstan motor which forms a portion of a servo loop. The capstan servo loop synchronizes the speed of the capstan motor with the vertical synchronization portion of the video signal, which is also employed to develop a 30 hertz signal that is recorded by a control track recording/reproducing head upon a control track located along an edge of the tape.
The above-mentioned prior-art recorder/reproducer further includes demodulation circuitry for use in decoding signals reproduced from a previous recorded tape. A color-under signal reproduced by the heads of the scanner is frequency demodulated to develop a reproduced luminance signal and frequency translated to develop a reproduced chrominance signal. Finally, the demodulation circuitry combines the reproduced luminance and reproduced chrominance signals to develop a reproduced video signal.
Although satisfactory for many applications, the above-mentioned prior-art recorder/reproducer is not designed to record/reproduce video signals so as to meet broadcast quality standards. More specifically, the performance of this prior-art recorder/reproducer is indirectly limited by the relatively low drum-carried-recording/reproducing-head-to-tape velocity, referred to as video-recording velocity, which is employed. The low video-recording velocity limits the upper frequency range of signals which may be recorded upon the tape, including the frequency-modulation portion of the color-under signal. Further, the frequency range of the frequency-modulated portion of the color-under signal is limited by the frequency-modulation process. The frequency-modulation process produces sideband energy which extends above and below the oscillator frequency in proportion to the band width of the modulating video signal. Should this energy extend down into the upper band width of the modulating video signal, undesirable Moire distortion will occur. Thus, the video band width of the above-mentioned prior-art recorder/reproducer is suitably limited. Since the band width of the video signal is limited, not only to a frequency less than that required to meet broadcast quality standards, but to a frequency too low to preserve the chrominance portion of the video signal, the above-mentioned prior-art recorder/reproducer processes the chrominance portion of the video signal separately, as a color-under signal.
A disadvantage particularly associated with processing the chrominance portion separately is that the luminance and chrominance portions of the video signal are recorded using carrier signals which are widely separated in frequency. As a result, undesirable relative time base instabilities are introduced with respect to the luminance and the chrominance portions of the video signal.
Further, the modulation index, which controls the deviation of the oscillator in response to the luminance signal, must be limited, resulting in a poor signal-to-noise ratio. Other disadvantages of the above-mentioned prior-art recorder/reproducer include its relatively low resolution and poor transient response.
Means for improving the performance of the above-mentioned prior-art recorder/reproducer are disclosed in my U.S. Pat. No. 4,214,259. Of immediate importance, the video recording velocity is tripled. The threefold increase in the recording velocity is obtained by so modifying the diameter of the pulleys which couple the drum-driving motor to the upper drum, that the drum is caused to rotate at a rate in excess of 5400 r.p.m. Additionally, to insure proper synchronization of the rotation of the upper drum with the horizontal-synchronization portion of the video signal, the frequency of the signal developed by the tachometer is reduced by a factor of three.
The increased video-recording velocity permits the color-under modulation and demodulation circuitry to be replaced by relatively wider band width frequency modulation and demodulation circuitry of the type which is known as wide band direct. The wide band direct modulation circuitry develops a signal suitable for recordation by the scanner upon the tape by modulating the frequency of a relatively higher-frequency oscillator responsive to the amplitude of the video signal, including both the luminance and chrominance portion thereof. The new circuitry permits video signals to be recorded/reproduced so as to meet broadcast quality standards.
It is important to note that when the rotational rate of the upper drum is increased, the drum and associate recording heads rotate at a rate which no longer permits time for each of the drum-carried recording/reproducing heads to record a complete field of the encoded video signal upon a single respective video track of the tape. Rather, each field of the encoded signal is divided, segmented, into portions each of which is recorded upon a respective one of several successive video tracks. Thus, for a threefold increase in the drum rotational rate, each field of the encoded video signal is segmented into three portions including a first portion that generally represents the upper third of the image and which is recorded upon one video track, a second portion that represents the middle third of the image and which is recorded upon the next track and a third, lower, portion which is recorded upon the following track.
Unmodified, the above-mentioned prior-art recorder/reproducer may be operated in a slow-motion mode simply by suitably reducing the rotational rate of the capstan. For example, decreasing the rotational rate of the capstan by a factor of four, causes the signal recorded upon each of the video tracks to be reproduced four times. Because a full field of the video signal is recorded upon each track, each field of the video signal is repeated four times producing a slow-motion effect.
Unfortunately, special problems are associated with the slow-motion reproduction of video tapes recorded in the segmented helical-scan format. Reducing the rotational rate of the capstan of a helical-scan recorder/reproducer modified for segmented operation causes segmented portions, rather than full fields, to be multiply reproduced. In other words, when the rotational rate of the capstan is reduced by a factor of four, for example, a field of the reproduced video signal may contain only the upper portion of the image repeated in the top, middle and lower positions. The next field may contain a top and two middle portions, and the following field will contain two middle and one lower portion and so on.